The present invention relates to energy generation systems and, more particularly, to a forced-circulation dual-phase reactor. A major objective of the present invention is to provide for enhanced core neutron power stability during pump shutdowns in a forced-circulation boiling-water reactor (FCBWR).
Nuclear reactors generate heat as a byproduct of fissioning in the reactor core, and generally remove this heat from the core using a liquid transfer medium. In dual-phase reactors, the core heat vaporizes the liquid transfer medium; this energy in the form of vapor pressure is readily transferred from the reactor for use elsewhere. The predominant type of dual-phase reactor is the boiling-water reactor (BWR). Accordingly, much of the following discussion concerning BWRs is readily extrapolated to other dual-phase reactors.
In a BWR, heat generated by nuclear fission in a core can be used to boil water to produce steam. Water passing through the core without being vaporized is recirculated within a reactor vessel to provide a continuous flow of water through the core. The steam that is generated can be separated from the water and transferred from the reactor vessel to deliver energy. For example, the steam can be used to drive a turbine, which in turn can be used to drive a generator to produce electricity. In the process, the steam condenses and can be returned to the vessel as feedwater. The condensate is merged with the internally recirculated water and continues to aid heat transfer.
BWRs can be distinguished by the means employed to recirculate the water within the reactor vessel. Forced-circulation boiling-water reactors (FCBWRS) rely primarily on pumps to drive the water along a recirculation path. Natural-circulation boiling-water reactors (NCBWRS) rely primarily on the driving force provided by the density difference between a downcomer and a steam column above the core. NCBWRs have the advantage of simplicity. However, their inherently lower pumping capacity limits reactor power output. Accordingly, the largest capacity BWRs are all FCBWRS. The distinction between FCBWRs and NCBWRs notwithstanding, FCBWRs are preferably designed to take advantage of natural circulation to allow decay heat to be removed from the core in the event the pumps are shut down.
In a NCBWR, water rising up from the core is guided vertically to promote steam-water separation and to support a relatively low-density steam/water head above the core. Water recirculates down the downcomer annulus between the reactor vessel and the chimney and core. The water in the downcomer is denser than the steam and water mixture in the core and chimney region. The difference in density forces circulation up through the core and chimney and down through the downcomer.
Natural circulation provides limited power output in part because its limited circulation rates allow the water flowing through the core more time than is optimal to be converted to steam. The excess boiling results in a larger volume of steam in the core. This larger steam volume adversely affects core stability, as the stability-decay ratio of the nuclear fission rate is dependent on the ratio of two-phase pressure drop to single-phase pressure drop. In NCBWRs, this problem is addressed by limiting the amount of heat generated by the core, and thus the power output of the reactor.
FCBWRs, on the other hand, are typically designed so that they exceed the power output that would be available using natural circulation alone. Total pumping power failure in an FCBWR operating at full capacity could result in excess boiling and core instability. To minimize the likelihood of total pumping power failure, several independent pumps are provided.
Despite the levels of safety afforded by redundant pumping, it is still worthwhile to enhance the throughput due to natural circulation in a FCBWR. Natural circulation is especially attractive as a safety backup because it does not depend on active components. Thus, improvements in natural circulation are highly desirable in FCBWRs.
The above-identified patent discloses the use of bypass valves to augment natural circulation in a FCBWR. The valves are designed to open automatically when the pumps stop pumping, thereby increasing the flow cross section and enhancing natural circulation. The valves are automatically closed while the pumps are operating to block backflow through the valves. The bypass valves have moving parts, which can give rise to reliability issues. An object of the present invention is to provide the advantages that these bypass valves provide for FCBWRs without requiring moving parts.